1. Technical Field
The present invention relates to wireless communications and, more particularly, polar transmitters in wireless communication devices.
2. Related Art
Modern wireless radio frequency (RF) transmitters for applications, such as cellular, personal, and satellite communications, employ digital modulation schemes such as frequency shift keying (FSK) and phase shift keying (PSK), and variants thereof, often in combination with code division multiple access (CDMA) communication. Independent of the particular communications scheme employed, the RF transmitter output signal, sRF(t), can be represented mathematically assRF(t)=r(t)cos(2πfct+θ(t))  (1)where fc denotes the RF carrier frequency, and the signal components r(t) and θ(t) are referred to as the envelope (amplitude) and phase of sRF(t), respectively.
Some of the above mentioned communication schemes have constant envelope, i.e.,r(t)=R, and these are thus referred to as constant-envelope communications schemes. In these communications schemes, θ(t) constitutes all of the information bearing part of the transmitted signal. Other communication schemes have envelopes (amplitudes) that vary with time and these are thus referred to as variable-envelope communications schemes. In these communication schemes, both r(t) and θ(t) constitute information bearing parts of the transmitted signal.
A common transmitter used in variable-envelope communication schemes is the polar transmitter. In a typical polar transmitter architecture, digital baseband data enters a digital processor that performs the necessary pulse shaping and modulation to some intermediate frequency (IF) carrier fIF to generate digital amplitude-modulated and digital phase-modulated signals. The digital amplitude-modulated signal is input to a digital-to-analog converter (DAC), followed by a low pass filter (LPF), along an amplitude path, while the digital phase-modulated signal is input to another DAC, followed by another LPF, along a phase path. The output of the LPF on the amplitude path is an analog amplitude (envelope) signal, while the output of the LPF on the phase path is an analog phase signal. The analog phase signal is input to a phase-locked loop (PLL) to enable the phase of an RF output signal to track the phase of the analog phase signal. The RF output signal is modulated in a power amplifier (PA) by the analog amplitude signal. Thus, in polar transmitter architectures, the phase component of the RF signal is amplified through the PA while the amplitude modulation is performed at the output of the PA.
In practice, the power spectrum emitted from a polar transmitter will not be ideal due to various imperfections in the RF transmitter circuitry. For example, one component of the RF circuitry that significantly affects the performance of the transmitter is the design of the low pass filter along the envelope path. Although metal oxide semiconductor (MOS) capacitors are more stable over process variations than traditional metal capacitors in filter designs, the filter resistor may still exhibit performance variations due to process and temperature changes, thus producing an undesirable variable filter response that can significantly affect the quality of the envelope signal. Therefore, what is needed is a polar transmitter that is capable of compensating for variations in the envelope low pass filter.